1. Field of the Invention
The present invention relates to a spark-ignition internal combustion engine, and particularly to a spark-ignition internal combustion engine comprising an engine body configured to have a high geometric compression ratio.
2. Description of the Background Art
Heretofore, in a spark-ignition internal combustion engine, there have been conducted various researches on a configuration of an engine body. For example, JP 2007-154827A discloses a result obtained through research on a height of a combustion chamber with a focus on collision between a flame and a combustion chamber wall. Specifically, if a flame prematurely collides with a combustion chamber wall, a flame growth rate becomes lower and thereby a combustion duration becomes longer, which leads to deterioration in engine thermal efficiency. With a view to suppressing such thermal efficiency deterioration, a spark-ignition internal combustion engine disclosed in the above patent publication is configured such that a spherical-shaped cavity (concave portion) is formed in a top surface of a piston at a position corresponding to a spark plug, to allow a region of a combustion chamber corresponding to the spark plug to be increased in height.
Meanwhile, it is conventionally known that a higher geometric compression ratio of an engine body provides enhanced engine thermal efficiency and improved fuel economy.
The conventional spark-ignition internal combustion engine disclosed in the above patent publication makes it possible to delay collision between a flame and a piston. However, in an engine body configured to have a high geometric compression ratio, if a height of a combustion chamber is simply increased to delay the collision, engine output power is liable to become insufficient.
Specifically, if it is attempted to increase the height of the combustion chamber while setting the geometric compression ratio of the engine body to a high value, a stroke length of the piston becomes longer along with the increase in height of the combustion chamber. If the stroke length becomes longer under the same engine displacement, a bore diameter of a cylinder becomes relatively smaller. If the bore diameter of the cylinder becomes smaller, a diameter of an intake valve provided in a cylinder head defining a ceiling of the combustion chamber becomes smaller and thereby an opening area of the intake valve becomes smaller. The smaller opening area of the intake valve is liable to cause failing to charge a sufficient amount of intake air during an increase in engine speed, resulting in lowering of engine output power.